Aadhaar number is mandatory for filing income tax returns from 1st July 2017. Those who do not have aadhaar number must apply for it and can quote aadhaar enrolment id. You can apply for aadhaar card here.

What is Aadhaar Enrolment id/number?

When you apply for Aadhaar you have to visit an enrolment Centre and fill the enrolment form. After giving demographic and biometric data and submission of proof of Identity and address documents you get an acknowledgement slip which has your Aadhaar Enrolment ID or Aadhaar Enrolment number.